In the field of hydraulic work machines such as hydraulic excavators, there have recently been proposed/developed hybrid-driven hydraulic work machines that are equipped with an electric assist motor (which is driven by an electricity storage device such as a battery) as a driving source other than the engine.
For example, a construction machine (hydraulic work machine) proposed in Patent Literature 1 is equipped with an electric assist motor driven by the engine. When there is a surplus in the engine output, the surplus engine output is stored as electric energy so as to achieve energy saving. When the engine output is insufficient, the electric motor is driven by discharging the stored electric energy so as to maintain necessary pump absorption torque. It is stated in the Patent Literature 1 that such a configuration enables employment of a small-sized engine (having rated output power equivalent to the average power necessary when the construction machine carries out operations), improvement in the fuel efficiency, and reduction in the CO2 emission.
In the construction machine proposed in the Patent Literature 1, when the workload is high and the requested power of the hydraulic pump (pump absorption torque) is greater than the rated power of the engine, assistance of the engine is carried out by performing the power running control on the electric assist motor to make the electric assist motor generate the differential power. Therefore, the hydraulic pump is enabled to generate power equivalent to that in conventional machines irrespective of the employment of a smaller-sized engine compared to those in the conventional machines of the non-hybrid-driven type. It is presumed in this construction machine that the engine continues generating the rated output power when the electric assist motor is assisting the engine. It is very important for the engine to continue generating the rated output power as will be explained below. However, no concrete method for enabling the engine to continue generating the rated output power is described in the Patent Literature 1.